Mývatn. At least I think this is the right trail. I walked it once ten or so years ago and after a few minutes' stroll found myself next to a fissure in a big head of lava that a rope ladder descended down into. We were to strip to our skivvies and climb down the ladder into the darkness of the lava formation, something I wasn't so sure about. Peer pressure won out, though, and I took one cautious step at a time into the unknown. About four or five feet down my toe dipped into water, warm water, pleasantly warm, hot tub temperature water, and as my eyes adjusted to the dark I realized that I was in an underground cavern about the size of an average kitchen, with a high ceiling and filled with very comfortable water. I was totally blown away! There was a slight ledge that I could almost sit on and light from outside to help reveal the rough design of this natural wonder. The only drawback was my constant worry that something was about to grab my toes and pull me under. But that's nothing new...I've had a phobia about lakes and their unseen inhabitants for ages. It's something about the stillness of the water that gets me. Give me a roiling ocean over a lake any day!
Anyway, I'm pretty sure this is the path we took to the underground pool. When I Googled Stóragjá I got this pretty cool picture and the key information that guides don't recommend dipping into the water because it's laden with bacteria due to slow water movement. So I guess it's another "at your own risk" Icelandic attraction. All I know is, I'm glad I did it, but I'm more glad I didn't know it was a bacteria soup before I went in!